Felling heads are widely used for the cutting and collecting of upright growing trees. In most cases, several tree trunks can be collected in the felling heads until the tree trunks are unloaded onto the ground. After this, the tree trunks are transported elsewhere by means of either a forwarder or a skidder. The boom system and the felling head are normally coupled to a high-power vehicle, such as a feller buncher, of which an example is given in U.S. Pat. No. 6,173,973 B1. Some felling heads are disclosed in documents US 2003/02025291 A1 and US 2002/0040741 A1. A felling head and a feller buncher are also disclosed in Canadian publication 2,196,728 A1. Vehicles provided with a horizontal movement of the boom are disclosed in publications U.S. Pat. No. 6,443,196 B1 and U.S. Pat. No. 6,997,221 B2.
The feller buncher normally comprises a lower carriage and an upper carriage. The lower carriage is equipped, for example, with two caterpillar tracks, by means of which the forest machine moves on a terrain. Above the traverser of the lower carriage, an upper construction is placed, comprising, inter alia, a cabin, an engine and an articulated boom construction. The boom construction has for example two parts, comprising a hoist boom and a stick boom, the felling head being articulated at the end of the stick boom.
The cabin of the feller buncher is provided with two control levers or control handles for the operator. The control levers are typically movable in two directions: forward-backward (y-axis) and to the left-right (x-axis). By means of the y-direction, one of the two control levers controls the position of the hoist boom (hoist function) and the other the position of the stick boom (stick function). In a corresponding manner, by means of the x-direction, one of the control levers typically controls the rotation of the upper carriage and the other the position of the felling head.
For bringing the felling head to the tree to be felled, the driver normally has to operate both of the control levers in the y-direction (hoist and stick functions). Because the movement of the hoist boom and/or the stick boom continuously changes the position of the felling head with respect to the ground, at least one of the control levers must also be moved in the x-direction. For felling the tree, the felling head is equipped with, for example, a saw that must be kept in the horizontal plane. A corresponding simultaneous operation of the control levers is required when the tree has been gripped in the felling head and the felling head together with the tree is moved towards new trees to be felled, or when a collected bunch of trees is unloaded onto the ground in a controlled manner for emptying the felling head. Consequently, in practice, the driver must almost all the time control said hoist and stick functions and the position of the felling head simultaneously, and moreover, he typically rotates the upper carriage. The control of the boom construction and the keeping of the felling head in the correct position require significant alertness and skill of the operator. The simultaneous manipulation of several control levers is difficult and may slow down the function.